Electron impact ion sources produce analyte ions by exposing analyte molecules to a focused electron beam. In conventional ion sources of this type, electrons are injected into the ionization chamber in a perpendicular direction with respect to the longitudinal axis of the ionization chamber (the ion exit axis, or z-axis). In this configuration, a substantial percentage of the ions are formed off of the ion exit axis, and thus only a reduced portion of ions passes to the mass analyzer for detection. In gas chromatography mass spectrometer (GC/MS) systems, there is the further difficulty that space charges of carrier gas ions can also impede the focusing of ions near the ion exit axis.
Ion sources have been developed in which collisions between ions and a damping gas reduce the phase space distribution of the ions and focus the ions near the z-axis, increasing the transmission of ions to the mass analyzer. Electrons may be injected either parallel or perpendicular to the quadrupole field using this source, while ions are extracted along the axis of the quadrupole field. However, in order to avoid injected electrons from reaching the entrance of the mass analyzer, the ionization chamber has a comparatively great length (typically greater than 60 millimeters) with a correspondingly large surface area. The large surface area of the ionization chamber makes it infeasible to use the source in the analysis of low concentrations of polarized chemical species. Furthermore, the large ionization volume of the source can be unsuitable in rapid GC/MS analyses because the gas residence time in the ionization chamber is close to or longer than the length of the detected peaks.
To address this problem, what is needed is an on-axis ion source having an ionization chamber with a reduced area that includes means for preventing injected electrons from reaching the entrance of the mass analyzer.